The present invention relates to a pressure measuring apparatus, more specifically, to a pressure measuring apparatus using a semiconductor pressure sensor.
It is known that a conventional semiconductor pressure sensor produces an output current that varies This property will be described with the use of FIGS. 8 to 10. First, the configuration of a conventional pressure measuring apparatus will be described with reference to the schematic diagram shown in FIG. 8. In FIG. 8, the conventional pressure measuring apparatus 2 has a sensor driving part 21, a pressure sensor part 22, and a voltmeter 23.
The sensor driving part 21 applies voltage to both ends of the pressure sensor part 22 for driving the pressure sensor part 22. The pressure sensor part 22 is a Wheatstone bridge impedance bridge circuit configured of four impedances. The voltmeter 23 measures the electric potential difference between the middle points in both arms of the impedance bridge circuit and it outputs and displays the measured value as sensor output.
Operation of the traditional pressure measuring apparatus will be described with reference to FIGS. 9 and 10. FIG. 9 illustrates a drive waveform output by the sensor driving part 21 for driving the pressure sensor part 22 and also indicates measurement sampling timing at which voltage is measured in the voltmeter 23 of the conventional pressure measuring apparatus 2. As shown in FIG. 9, the drive waveform output by the sensor driving part 21 is a step waveform, and the electric potential difference between the middle points in both arms of the pressure sensor part 22 is measured at predetermined time intervals (timing).
FIG. 10 is a diagram illustrating the sensor output current-carrying property of the pressure sensor part 22 of the conventional pressure measuring apparatus 2. As shown in FIG. 10, the output of the pressure sensor part 22 has a transition period portion that is a fixed period from voltage application by the sensor driving part 21 and a steady period state period portion after the fixed period of the transition period portion. Thus, the semiconductor pressure sensor has a sensor output variation with a transit phenomena. On this account, various methods have been used traditionally to overcome this. Such methods include, for instance, measuring voltage a plurality of times from the beginning of measurement sampling, calculating the average value of each measured value, and using this as the measured value this time, or measuring the voltage at predetermined times or at predetermined times after a fixed period of time has passed and taking the average value as the measured value.
However, the traditional pressure measuring apparatus 2 has the transition period of pressure variation due to current carrying, and thus it can be considered that pressure is measured at a fixed point (time) from current carrying, but it has had a problem that the initial state is varied in the case of the midway returning from the steady state to the initial state after once pressure has been measured and thus accurate values cannot be measured.
In addition, in the conventional pressure measuring apparatus 2, time for the transition period of pressure variation due to current carrying shown in FIG. 10 sometimes takes several tens of seconds, which causes the problem that measurement is performed while sensor output is varying for a predetermined plurality of times for measurement sampling.
Furthermore, in order to solve these problems, when current is carried until the output variation in the pressure sensor part 22 becomes stable and then pressure is measured, consumption current carried through the pressure sensor part 22 becomes greater. In this manner, the amount of electric power consumption of the pressure measuring apparatus 2 is increased. Therefore, even though the housing is allowed to be smaller, there has been a problem that the portability of the traditional pressure measuring apparatus 2 is impaired in relation to electric power consumption.
The present invention has been made in view of the foregoing problems. An object of the invention is to provide a portable pressure measuring apparatus with a pressure sensor of highly accurate output and small electric power consumption.
In order to solve the foregoing problems, the pressure measuring apparatus of the invention comprises an impedance bridge circuit comprised of a plurality of impedance elements arranged in a bridge configuration for measuring pressure, a drive unit for applying voltage to the impedance bridge circuit, a measuring unit for sampling and measuring the electric potential difference between the middle points in both arms of the impedance bridge circuit at a fixed time intervals, and a determining unit for determining whether the difference between the current pressure and a previous pressure sampled and measured by the measuring unit reaches a predetermined threshold or below.
According to the present invention, pressure is sampled and measured by the measuring unit at predetermined timing, the difference between the successively measured values is determined by the determining unit to determine whether the difference has reached the predetermined threshold or below, and the measured value that has reached the threshold or below is displayed, output, and the like, as a sensor output. Accordingly, the output accuracy of the pressure sensor in the pressure measuring apparatus can be enhanced because of a reduced influence of the transition phenomena due to the output variation in the pressure sensor.
In addition, it is unnecessary to perform measurement sampling for a considerable number of times in order to increase the output accuracy of the pressure sensor as previously done, and thus, the electric power consumption of the pressure measuring apparatus can be suppressed. As a result, the apparatus does not need electric power supply from an external power source and it is portable.
Furthermore, as the pressure sensor applied to the invention, a semiconductor pressure sensor, in which four polysilicon piezoresistors are used to form a Wheatstone bridge, and the like can be considered, but it is not limited to these. Any pressure sensor having a property of output variation is acceptable.
Moreover, in the present invention, it is acceptable that a drive voltage waveform applied by the above drive unit is either a step waveform or pulse waveform. The unique advantage of the invention is also apparent when the drive voltage is a step voltage. However, in the case where the drive voltage is a pulse voltage, the output of the pressure sensor can returned to a steady state quicker than in the case of a step voltage. More specifically, time for transition phenomena can be shortened and electric power can be reduced in the pulse waveform more than in the step waveform, and thus the number of times for measurement sampling can be decreased and the consumption current (electric power consumption) of the pressure sensor can be further reduced.
Besides, in order to solve the foregoing problems, the pressure measuring apparatus of the invention comprises an impedance bridge circuit for measuring pressure, a drive unit for applying voltage in a pulse drive waveform to the impedance bridge circuit, and a measuring unit for sampling and measuring the electric potential difference between the middle points in both arms of the impedance bridge circuit at a fixed time intervals.
According to the invention, the drive unit applies the voltage in the pulse drive waveform to the pressure sensor formed of the impedance bridge circuit, whereby the output of the pressure sensor can be turned to the steady state soon. Thus, the number of times for measurement sampling can be decreased, whereby the consumption current of the pressure sensor can be reduced, that is, the electric power consumption of the pressure measuring apparatus can be reduced.